The present application relates to an instrument and method for determining the optical absorption of matter, including black carbon, in the atmosphere.
Black carbon (BC), or soot, is a primary light-absorbing component of particulate matter generated by biomass and fossil fuel combustion. BC is extremely harmful to both human health and the environment. It has been estimated that airborne particulate matter, including BC, kills an estimated 4 million people per year worldwide. In addition, BC is the second largest contributor to global warming after carbon dioxide. In order to assess BC's impact on climate and human populations, it is necessary to measure both the geographical and vertical distribution of BC in the atmosphere on a large scale.
In the past, an instrument known as an Aethalometer has been employed to measure the concentration of optically absorbing (“black”) suspended particulates in a gas. Such airborne particulate matter (PM) is commonly visualized as smoke or haze and is often seen in ambient air under polluted conditions. Current Aethalometers or BC instruments are usually large bench-top units and are quite expensive to acquire. Furthermore, current BC instruments are not designed to be deployed outdoors and, thus, perform poorly in fluctuating ambient conditions.
In the past, BC instruments have been devised. For example, U.S. Pat. No. 4,893,934 describes an Aethalometer having a single light source and a single light detector that produces two light paths. A filter collects particles from ambient air in a collection area. A rotating disc allows light from a source to pass alternately through the two light paths and, thus, through the collection and reference areas of the filter. The intensity of light transmitted through the filters are measured and compared to determine the absorption coefficient of the aerosol particles collected on the filter.
U.S. Pat. No. 7,038,765 describes an apparatus and method for measuring black carbon in the atmosphere which utilizes a filter tape that is continually illuminated by a source. Light absorbing aerosol material collected on the tape is continually determined from changes in the optical properties of the tape collection areas, with the aid of known algorithms.
U.S. Pat. No. 9,018,583 shows a method for automatically diagnosing and calibrating a photometric particle analyzer that employs a trio of sensors receiving light from a light source through a filter having a collection and reference portion to produce first and second signals. A third sensor produces a third signal without passing through the filter. Thus, baseline and test measurement comparisons are obtained to provide a diagnostic of the operation of the photometric particle analyzer.
An article entitled “Calibration and Intercomparison of Filter Based Measurements of Visible Light Absorption By Aerosols”, Aerosol Science and Technology, Vol. 30, pp. 582-600, 1999, Bond et al., shows an analyzer where the differences between sequential optical measurements are used to estimate particular concentrations of various types.
United States Patent Publication 2016/0349175 teaches an apparatus receiving an analyte in which two housing portions are clamped together sandwiching a substrate to which analyte is delivered. Optical signals of various wavelengths are passed through the substrate to study analyte properties and are analyzed by optical sensors.
United States Patent Publication 2017/0370809 shows a portable air sampling device that utilizes a filter receiving and trapping airborne particles. The average mass concentration of aerosol collected by the filter is determined using gravimetric analysis.
A low cost, compact instrument for measuring the optical absorption of and concentration of matter, such as particulate black carbon pollutants and other analytes, is needed. Such an instrument should be easy deployed on a massive scale to measure BC distributions both on the ground and in the atmosphere, accurately and reliably under fluctuating environmental conditions.